The short answer is: it doesn't. Reflection takes place when some part of a wave bounces off of an interface between two media back into the original medium (medium = stuff that the wave is traveling through).

Any light that isn't emitted by an object is reflected light. This is key to being able to see materials that don't emit light. Reflection of light occurs in a very predictable manner. When light reaches a smooth surface, that angle at which it bounces off is the same as that at which it hits the surface. This is called the law of reflection. That being said, there are two categories of reflection:
specular and diffuse.

Specular reflection takes place when light rays are reflected from a surface in essentially the same direction. This occurs for a objects which are nearly planar over a large area and is the type of reflection which produces recognizable images (such as looking at yourself in a mirror). Diffuse reflection is when the light reflected from a surface is directed in many misaligned angles. Diffuse reflection may seem to contradict the law of reflection, but it is actually in accordance with it.

The surfaces of most materials are not smooth. Rather, they comprise a large number of small flat surfaces which are all oriented differently. Light follows the law of reflection at each of these surfaces, resulting in a different angle for the outgoing ray.

The word transparent is often taken to mean that all light that hits an object will pass through. However, "transparent" objects are usually just mostly transparent; although most of the light passes through, some is reflected. This is why windows, water, and other "transparent" materials can reflect light.

The amount of light that is reflected depends on the angle at which the light strikes the surface and a property called the refractive index. The fraction that is reflected can be calculated from these quantities using the Fresnel equations.

For some additional information, look at these answers on ScienceLine [How reflections happens; "perfect" reflections; and why some materials reflect and others don't.]

Actually reflection occurs in most materials. An original light ray, called the “incident” ray, comes in, hits a flat surface like a window or a mirror or a wall, and reflects off at the same angle it came in. Some of the best reflecting materials are not transparent! For example, silver and aluminum are very reflective materials and are used in mirrors. Dark red paint and red brick are really bad reflecting materials.

Polished metal is not transparent, and yet does create a reflection. Reflection therefore can occur on objects which are not transparent.